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United States Patent |
6,257,058
|
Murata
|
July 10, 2001
|
Silicon gyroscope and method of driving the same
Abstract
A silicon gyroscope comprises a vibrator 1 which is made from a silicon
wafer and has three elastic arms 2 separated by two notches, with part of
the root section of vibrator which supports the elastic arms being fixed
to a base plate 3, horizontal electrodes 4 which are laid between the
elastic arms and on the outer sides of the elastic arms by being
orthogonal to the plane of layout of the elastic arms and close to the
elastic arms, and vertical electrodes 5 which are laid in correspondence
to the elastic arms by being parallel to the arm layout plane and close to
the elastic arms. The silicon gyroscope which is driven in innovative
manner is capable of stably detecting the angular velocity.
Inventors:
|
Murata; Shinji (Iwate-ken, JP)
|
Assignee:
|
Alps Electric Co., Ltd. (Tokyo, JP)
|
Appl. No.:
|
298746 |
Filed:
|
April 23, 1999 |
Foreign Application Priority Data
| Apr 27, 1998[JP] | 10-117308 |
| Sep 14, 1998[JP] | 10-260392 |
Current U.S. Class: |
73/504.16; 310/370 |
Intern'l Class: |
G01P 009/04 |
Field of Search: |
73/504.12,504.14,504.15,504.16,504.04,504.02,514.32
310/370,321
|
References Cited
U.S. Patent Documents
4598585 | Jul., 1986 | Boxenhorn.
| |
5451828 | Sep., 1995 | Tomikawa et al.
| |
5461916 | Oct., 1995 | Fujii et al. | 73/504.
|
5691595 | Nov., 1997 | Tomikawa et al. | 310/370.
|
5708320 | Jan., 1998 | Ohnishi et al. | 310/370.
|
5756895 | May., 1998 | Kubena et al. | 73/504.
|
5824900 | Oct., 1998 | Konno et al. | 73/504.
|
5861705 | Jan., 1999 | Wakatsuki et al. | 73/504.
|
6003372 | Dec., 1999 | Kawakami et al. | 73/504.
|
Primary Examiner: Kwok; Helen
Attorney, Agent or Firm: Brinks Hofer Gilson & Lione
Claims
What is claimed is:
1. A silicon gyroscope comprising a vibrator, a base plate, horizontal
electrodes, and vertical electrodes, said vibrator being made from a
single crystal silicon and having a base portion, with three elastic arms
extending from said base portion and separated by two notches, and said
base plate being formed from glass, wherein:
part of a root section of the base portion of said vibrator is fixed to
said base plate;
said horizontal electrodes are formed of a silicon wafer, and are disposed
between said elastic arms and on outer sides of said elastic, said
horizontal electrodes being orthogonal to a plane formed by the layout of
said elastic arms and positioned adjacent to said elastic arms in such a
manner that part of said horizontal electrodes contact said base plate;
said vertical electrodes are formed of a metallic film, and include two
electrodes which are disposed in a groove formed on an upper surface of
said base plate and to which alternating voltages having phases inverted
to each other are applicable by being parallel to the layout plane of the
elastic arms and adjacent to said elastic arms; and
said horizontal electrodes and said vertical electrodes being disposed in
directions orthogonal to each other.
2. A silicon gyroscope comprising a vibrator which is made from a silicon
wafer and having three elastic arms separated by two notches, with part of
a root section of the vibrator supporting said elastic arms being fixed to
a base plate; and
vertical electrodes which are laid in correspondence to said elastic arms
by being parallel to a plane formed by the layout of said elastic arms and
disposed adjacent to said elastic arms, wherein said elastic arms are
dimensioned to have a thickness d and a notch width W related in terms of
the value of W/d to be in the range from 1 to 0.02,
wherein said vertical electrodes comprise driving electrodes, Coriolis
force detecting electrodes, and drive synchronous electrodes for detecting
the Coriolis force in the vertical direction, said driving electrodes
being located in pairs to confront each of said elastic arms, said
Coriolis force detecting electrodes being located to confront each of said
notches and outer sides of said elastic arms, and said drive synchronous
electrodes being located between each of said pair of driving electrodes.
3. A silicon gyroscope according to claim 2 further including horizontal
electrodes which are positioned adjacent to the outer sides of said
elastic arms by being orthogonal to the plane formed by the layout of said
elastic arms.
4. A method of driving a silicon gyroscope, said silicon gyroscope
comprising a vibrator which is made from a silicon wafer and has three
elastic arms separated by two notches, with part of a root section of the
vibrator supporting said elastic arms being fixed to a base plate, and
vertical electrodes which are laid in correspondence to said elastic arms
by being parallel to a plane formed by the layout of said elastic arms and
disposed adjacent to said elastic arms, said vertical electrodes
comprising driving electrodes, Coriolis force detecting electrodes, and
drive synchronous electrodes for detecting the Coriolis force in the
vertical direction, said driving electrodes being located in pairs to
confront each of said elastic arms, said Coriolis force detecting
electrodes being located to confront each of said notches and outer sides
of said elastic arms, and said drive synchronous electrodes being located
between each of said pair of driving electrodes, wherein said method
comprises applying voltages consisting of d.c. components of opposite
polarities and a.c. components of opposite phases to each said pair of
driving electrodes.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a silicon gyroscope for detecting the
rotatory angular velocity based on the Coriolis force which arises when
the vibrator of the gyroscope turns while vibrating, and also relates to a
method of driving the silicon gyroscope. Particularly, the invention
relates to a silicon gyroscope which is capable of stably detecting the
angular velocity and a silicon gyroscope which has enhanced sensitivity
without the need of supply of a high drive voltage, and also relates to a
method of driving the silicon gyroscopes.
2. Description of the Prior Art
There has been developed recently a compact vibratory gyroscope for use in
a navigation system equipped on motor vehicles, an attitude controller of
unmanned vehicles, and a field swing preventive device of video cameras.
This vibratory gyroscope consists of a vibrator having three parallel
elastic arms separated by two notches, a drive means for vibrating the
elastic arms, and a means of detecting the vibration component which is
orthogonal to the direction of vibration of the elastic arms resulting
from the turning of the vibrator. The vibrator of the gyroscope is made of
piezoelectric ceramics or constant elasticity metal (elinvar).
The vibrator of piezoelectric ceramics necessitates only the formation of
electrodes for current conduction by basing the operation on its own
piezoelectricity, and it can be simple in structure. However,
piezoelectric ceramics generally has a small Q value of vibration (ranging
from 20 to 1000). Therefore, it cannot be expected to have a large
resonance-based displacement amplification effect. It also has drawbacks
of the need of large input energy and the heating caused by the large
input energy.
The piezoelectric ceramics material also has a large temperature-dependent
variation of Young's modulus (ranging from 10 to 200 ppm) and a large
linear expansion coefficient (ranging from 10 to 50 ppm). On this account,
the vibrator of piezoelectric ceramics varies in its dimensions in
response to the variation of environmental temperature, resulting possibly
in a sensor output variation. It also suffers from the temperature
dependence of piezoelectricity (ranging from 100 to 5000 ppm) and thus can
have output fluctuation in response to the variation of environmental
temperature even in the absence of variation of dimensions.
Moreover, the piezoelectric sensor heats up by itself, and therefore it is
liable to have output fluctuation during the period after it is turned on
until it reaches the steady state temperature even at a constant
environmental temperature. For example, for a gyroscope sensor which bases
the azimuth detection on the integration of angular velocity output, a
fluctuation of null value of the output results in an error of detected
azimuth.
In the case of a vibrator formed of constant elasticity metal,
piezoelectric elements are glued to the elastic arms and energized so that
the arms vibrate, causing the whole vibrator of constant elasticity metal
to vibrate. This vibrator is more complex in structure than the vibrator
of piezoelectric ceramics mentioned previously.
FIG. 35 is a perspective view of a vibrator of a conventional gyroscope
made from constant elasticity metal, and FIG. 36 is a cross-sectional view
of the vibrator arm. The vibrator 200 has elastic arms 201a,201b and 201c
which extend in parallel to each other, on which are glued piezoelectric
elements 202a-202l, with electrodes 203a-203l for current conduction being
formed thereon.
The vibrator of constant elasticity metal, with the piezoelectric elements
202 being glued thereon, has problems similar to the above-mentioned
problems inherent to piezoelectric ceramics, and further has a problem of
the divergence of resonant frequencies of drive and detection caused by
the distortion of vibrator 200 due to different thermal expansion
coefficients of different materials glued together, a problem of output
fluctuation caused by the variation of vibration amplitude of the vibrator
200, a problem of the influence of vibration of the vibrator 200 when
there is a gap between the piezoelectric element 202 and the vibrator 200,
and a problem of the influence on the output signal attributable to the
Coriolis force.
In regard to the conventional drive method for the. silicon gyroscope, a
voltage is applied between the vibrator and the driving electrodes in the
vibrator drive direction so that the vibrator is driven by the
electrostatic force, and the value of displacement of the vibrator caused
by the Coriolis force is detected in terms of the variation of static
capacitance between the vibrator and the detecting electrodes.
However, this conventional drive method not only needs to deal with a small
static capacitance (0.1 to 3 pF) in a quiescent state, but also an
extremely small variation of static capacitance (5 to 500 aF) arising in
response to a displacement of vibrator caused by the Coriolis force. In
addition, a C-V conversion circuit which converts the static capacitance
into a voltage value is extremely susceptible to external noises
attributable to electromagnetic induction or the like due to a high input
impedance of the circuit. Moreover, due to the device structure in which
the driving electrodes and detecting electrodes are located closely on the
vibrator, it is difficult to prevent the induction noise created by the
driving electrodes from leaking to the high-impedance detecting
electrodes.
The static capacitance varies in response to the Coriolis force at the same
frequency as driving of the vibrator, making it difficult to separate the
signal from noise and thus causing the C-V conversion circuit to have a
smaller gain, resulting in a smaller sensitivity of detection of angular
velocity.
The conventional drive method bases the flexure of vibrator on the
expansion and contraction of piezoelectric element, and therefore the
vibrator is liable to twist in the motion of drive vibration due to the
unevenness of piezoelectric material. Similarly, at detection, the
displacement of vibrator created by the Coriolis force in the direction
orthogonal to the direction of drive vibration fluctuates. Error of
orthogonality between the vibration direction of drive and the vibration
direction of detection creates a mechanical coupling of the driving signal
with the vibration of detection, and affects the output signal, resulting
in a drift or offset of the output signal.
From the foregoing viewpoints, the conventional gyroscope and the
associated drive method are deficient seriously in implementing the stable
detection of angular velocity.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a silicon gyroscope and
a method of driving the same which are capable of stably detecting the
angular velocity.
Specifically, the inventive silicon gyroscope comprises a vibrator which is
made from a silicon wafer and has three elastic arms separated by two
notches, with part of the root section of the vibrator which supports the
elastic arms being fixed to a base plate, horizontal electrodes which are
laid between the elastic arms and on the outer sides of the elastic arms
by being orthogonal to the plane of layout of the elastic arms and close
to the elastic arms, and vertical electrodes which are laid in
correspondence to the elastic arms by being parallel to the arm layout
plane and close to the elastic arms.
In the inventive silicon gyroscope, the horizontal electrodes are made from
a silicon wafer, the vertical electrodes are made from a metallic film,
and the base plate is a glass plate, on which the horizontal electrodes
and the vertical electrodes are formed. The vertical electrode for each
elastic arm consists of two electrodes, to which are applied voltages of
opposite polarities.
In regard to this object of the invention, the vibrator is formed of
monocrystalline silicon so that a large Q value (ranging from 1000 to
20000) is obtained, thereby producing a stable output throughout the
long-term use in the environment of varying temperature. Moreover, in
regard to this object of the invention, the vibrator is rid of attached
members of different material so that the vibration of vibrator is free
from their influence. Silicon has as small thermal expansion coefficient
as 2 ppm, which less affects the resonant frequency, and in the case of
employing a self oscillation circuit, it varies the oscillation frequency
in response to the expansion and contraction of the vibrator, thereby
retaining the resonant state.
Another object of the present invention is to provide a silicon gyroscope
which has enhanced sensitivity without the need of supply of a high drive
voltage based on the reduction of distance between elastic arms, and yet
which is inexpensive. Specifically, the inventive silicon gyroscope
comprises a vibrator which is made from a silicon wafer and has three
elastic arms separated by two notches, with part of the root section of
vibrator which supports the elastic arms being fixed to a base plate, and
vertical electrodes which are laid in correspondence to the elastic arms
by being parallel to the plane of layout of the elastic arms and close to
the elastic arms, wherein the elastic arms are dimensioned to have a
thickness d and a notch width W related in terms of the value of W/d to be
in the range from 1 to 0.02.
In the inventive silicon gyroscope, the vertical electrodes are designed to
consist of driving electrodes provided in correspondence to individual
elastic arms and Coriolis force detecting electrodes which confront the
notches and the outer sides of the elastic arms, horizontal electrodes are
laid on the outer sides of the elastic arms by being orthogonal to the
direction of layout of the elastic arms and close to the elastic arms, the
driving electrodes are designed to consist of two electrodes, and a
drive-synchronous electrode for detecting a Coriolis force in the vertical
direction is formed between the two driving electrodes.
In regard to this object of the invention, horizontal electrodes which are
laid between the elastic arms originally are eliminated thereby to reduce
the distance between the elastic arms so that a large Q value (ranging
from 1000 to several tens thousands) is obtained. Moreover, in regard to
this object of the invention, the Coriolis force in the horizontal
direction is detected based on the detection of the variation of the
coincidence factor between the elastic arm and Coriolis force detecting
electrode, with the accuracy of horizontal Coriolis force detection being
enhanced by the provision of the horizontal electrodes on the outer sides
of the elastic arms. Moreover, in regard to this object of the invention,
the driving electrodes are used to detect the vibration of vertical
direction of the vibrator.
Still another object of the present invention is to provide a method of
driving a silicon gyroscope which includes two vertical electrodes or
driving electrodes laid in correspondence to elastic arms of a vibrator,
with voltages including d.c. components of opposite polarities and a.c.
components of opposite phases being applied in pairs to the two vertical
electrodes or driving electrodes.
In regard to this object of the invention, the mutual influence of
electrodes are cancelled by the application of d.c. components of opposite
polarities and a.c. components of opposite phases to the respective
electrodes, thereby improving significantly the sensitivity of detection
of angular velocity.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a silicon gyroscope based on a first
embodiment of this invention;
FIG. 2 is a plan view of the silicon gyroscope shown in FIG. 1;
FIG. 3 is a cross-sectional view taken along the line 3--3 of FIG. 2;
FIG. 4 is a cross-sectional view taken along the line 4--4 of FIG. 2;
FIG. 5 is a flowchart showing the fabrication process of the silicon
gyroscope shown in FIG. 1;
FIG. 6 is a block diagram of the drive/detect circuit of the silicon
gyroscope of the first embodiment;
FIG. 7 is a graph showing the output characteristics of the amplify/d.c.
voltage merge circuit 119 shown in FIG. 6;
FIG. 8 is a graph showing the output characteristics of the amplify/d.c.
voltage merge circuit 118 shown in FIG. 6;
FIG. 9 is a conceptual diagram showing the displacements of elastic arms at
time point t=T in FIG. 7 caused by driving;
FIG. 10 is a conceptual diagram showing the displacements of elastic arms
at time point t=T in FIG. 7 caused by the Coriolis force;
FIG. 11 is a conceptual diagram showing the displacements of elastic arms
at time point t=T in FIG. 7 caused by driving with the application of
opposite-phase voltages;
FIG. 12 is a conceptual diagram showing the displacements of elastic arms
at time point t=T in FIG. 7 caused by the Coriolis force with the
application of opposite-phase voltages;
FIG. 13 is a perspective view of a silicon gyroscope based on a second
embodiment of this invention;
FIG. 14 is a plan view of the silicon gyroscope shown in FIG. 13;
FIG. 15 is a cross-sectional view taken along the line 15--15 of FIG. 14;
FIG. 16 is a cross-sectional view taken along the line 16--16 of FIG. 14;
FIG. 17 is a block diagram of the drive/detect circuit of the silicon
gyroscope of the second embodiment;
FIG. 18 is a graph showing the output characteristics of the amplify/d.c.
voltage merge circuit 135 shown in FIG. 17;
FIG. 19 is a graph showing the output characteristics of the amplify/d.c.
voltage merge circuit 134 shown in FIG. 17;
FIG. 20 is a graph showing the output characteristics of the amplify/d.c.
voltage merge circuit 132 shown in FIG. 17;
FIG. 21 is a graph showing the output characteristics of the amplify/d.c.
voltage merge circuit 131 shown in FIG. 17;
FIG. 22 is a conceptual diagram showing the displacements of elastic arms
at time point t=T in FIGS. 18 to 21 caused by driving;
FIG. 23 is a conceptual diagram showing the displacements of elastic arms
at time point t=T in FIGS. 18 to 21 caused by the Coriolis force;
FIG. 24 is a conceptual diagram showing the displacements of elastic arms
at time point t=T in FIGS. 18 to 21 caused by driving with the application
of opposite-phase voltages;
FIG. 25 is a conceptual diagram showing the displacements of elastic arms
at time point t=T in FIGS. 18 to 21 caused by the Coriolis force with the
application of opposite-phase voltages;
FIG. 26 is a plan view of a silicon gyroscope based on a third embodiment
of this invention;
FIG. 27 is a cross-sectional view taken along the line 27--27 of FIG. 26;
FIG. 28 is a cross-sectional view taken along the line 28--28 of FIG. 26;
FIG. 29 is a block diagram showing an embodiment of the drive/detect
circuit of the silicon gyroscope shown in FIG. 26;
FIG. 30 is a block diagram showing another embodiment of the drive/detect
circuit of the silicon gyroscope shown in FIG. 26;
FIG. 31 is a plan view of a silicon gyroscope based on a fourth embodiment
of this invention;
FIG. 32 is a cross-sectional view taken along the line 32--32 of FIG. 31;
FIG. 33 is a cross-sectional view taken along the line 33--33 of FIG. 31;
FIG. 34 is a graph showing the relation between the width of gap of elastic
arms and the Q value of vibrator;
FIG. 35 is a perspective view of a conventional vibratory gyroscope made
from constant elasticity metal; and
FIG. 36 is a cross-sectional view taken along the line 36--36 of FIG. 35.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Embodiments of the present invention will be explained with reference to
the drawings.
The main body of silicon gyroscope of the first embodiment shown in FIG. 1
and FIG. 2 is made up of a vibrator 1, a base plate 3, horizontal
electrodes 4, and vertical electrodes 5. The vibrator 1 which is made of
silicon consists of a root section la and three elastic arms 2a, 2b and 2c
which are parallel to each other and separated by two notches running from
the root section la. The vibrator 1 is joined at its end of root section
la to the base plate 3 of glass by means of anode plate bonding technique
or the like.
The horizontal electrodes 4 of this embodiment are made of silicon, and are
laid between the individual elastic arms 2a,2b and 2c and on the outer
sides of the elastic arms 2a and 2c by being orthogonal and close to these
arms. The individual horizontal electrodes 4a,4b,4c and 4d are formed on
the base plate 3.
The vertical electrodes 5 of this invention is made of chrome, and the
individual vertical electrodes 5a,5b and 5c are laid in grooves 3a, which
are formed on the upper surface of the base plate 3, by being parallel to
the direction of layout of the elastic arms 2a,2b and 2c and close to the
arms.
The vibrator 1 is adapted to vibrate at its elastic arms 2a,2b and 2c by
being secured at its root section la. The vibration of the elastic arms
2a,2b and 2c propagated to the root section la is so weak that it does not
virtually cause the end of root section la to vibrate. According to this
invention, the end of root section la is joined to the base plate 3 so
that it does not affect the vibration of the elastic arms 2a,2b and 2c.
FIG. 2 shows by a plan view the dimensions of the vibrator 1 shown in FIG.
1. The vibrator 1 fabricated to have these dimensions operates at a
resonant frequency of about 2 kHz. FIG. 3 and FIG. 4 are cross-sectional
views taken along the line 3--3 and line 4--4, respectively, of FIG. 2.
As shown in these figures, the vertical electrodes 5 and horizontal
electrodes 4 of this embodiment are laid in the orthogonal directions. The
vibrator 1 can vibrate freely in the vertical and horizontal directions,
so that it vibrates without twisting by being driven in the horizontal or
vertical direction, allowing the detection of only the displacement
component orthogonal to the drive direction caused by the Coriolis force
acting on the vibrator 1 in the direction orthogonal to the drive
direction.
The vibrator 1 of this embodiment is made of monocrystal silicon having
little defects of lattice, so that there is no distortion in it and it has
superior thermal characteristics. The vibrator 1 is designed to operate
entirely in non-contact manner for driving and angular velocity detection,
and it has no additional members attached which can affect the vibration
of vibrator 1, thereby enabling the stable detection of angular velocity.
The material of the vibrator 1 is high-purity monocrystal silicon, allowing
it to have a large Q value (ranging from 1000 to 20000). In addition,
because of extremely little defects of lattice included in the material,
the vibrator 1 is free from the vibration-caused fatigue, the plastic
deformation even under the large-amplitude vibration, and the holding of
distortion and internal stress in the silicon material, whereby it can
produce a stable output throughout the long-term use in the environment of
varying temperature.
The vibrator 1 of the inventive silicon gyroscope has no attachment of
different material, allowing itself to vibrate stably without being
affected by other members, and it is free from the creation of gaps which
would occur at the formation of piezoelectric elements or electrodes on
the vibrator 1 and free from the exertion of a distortion or stress to it.
The vibrator 1 is free from the distortion attributable to different
thermal expansion coefficients of different materials, and free from the
influence of temperature variation associated with the different
materials.
Silicon has as small thermal expansion coefficient as 2 ppm, which less
affects the resonant frequency as compared with piezoelectric elements. In
the case of employing a self oscillation circuit, it varies the
oscillation frequency in response to the expansion and contraction of the
vibrator 1, thereby retaining the resonant state.
The vibrator 1 of silicon can be fabricated precisely on a mass-production
scale based on the photolithography. The resulting accurate orthogonality
minimizes the influence of the drive vibration on the vibration of
detection, and eventually prevents the influence on the output signal.
FIG. 5 shows by a flowchart the fabricating process of the inventive
silicon gyroscope. The process consists of four major stages, which are a
process of a glass plate which will become the base plate 3, a process of
silicon which will become the vibrator 1, a process of joining the
vibrator 1 to the base plate 3, and a process of the vibrator 1.
A glass plate for the base plate 3 is rendered the patterning of step ST1
and the subsequent etching of step ST2 for forming grooves 3a on the upper
surface of the glass plate, so that a gap of about 20 pm is created
between the vibrator 1 and the bottom of grooves 3a.
Subsequently, a chrome film of about 0.5-.mu.m thickness is formed by
sputtering in step ST3 and next patterned in step ST4, so that the
vertical electrodes 5 of the intended shape are formed on the bottom of
grooves 3a.
In the silicon process, an insulation film is formed on the entire surface
of a silicon wafer in step ST11, and it is patterned for diffusion in step
ST12 and rendered the diffusion of B (boron) for ohmic contact in step
ST13. The position of ohmic contact on the root section la is selected to
be above the joint section against the glass base plate 3 so that it does
not affect the vibration of vibrator 1.
Subsequently, the insulation film is removed in step ST14, another
insulation film which serves for the mask in forming the vibrator 1 and
horizontal electrodes 4 is formed in step ST15, and it is patterned in
step ST16. Next, contact holes for a signal lead-out Cr film is formed in
step ST17, the signal lead-out Cr film is formed in step ST18, and it is
patterned to leave only the electrode section in step ST19.
The resulting silicon member is joined to the glass base plate 3 by use of
anode plate bonding technique in step ST21. Subsequently, notches for
separation are formed by etching in step ST22 for the formation of the
vibrator 1 and horizontal electrodes 4.
Finally, the insulation film is removed, the wafer is divided into chips of
a certain size by means of a dicer or the like, and the chips are planted
and wired by bonding or the like on circuit boards, although these
processes are not shown on the flowchart of FIG. 5.
FIG. 6 shows by a block diagram the circuit arrangement for driving and
detection of the silicon gyroscope of the first embodiment. An oscillation
circuit 101 generates an alternating voltage having a frequency of 50 kHz
and amplitude of 5 volts, and the voltage is applied to the vertical
electrodes 5a and 5c that confront the outer elastic arms 2a and 2c of the
vibrator 1. A phase inverting circuit 102 produces a carrier wave of
opposite phase relative to the voltage provided by the oscillation circuit
101, and it is applied to the vertical electrode 5b which confronts the
middle arm 2b.
The vertical electrodes 5a, 5b and 5c are spaced out with a gap of 20 .mu.m
from the elastic arms 2a,2b and 2c, respectively, of the vibrator 1, so
that a static capacitance emerges across the gaps, which is converted into
a voltage value by a C-V conversion circuit 103.
The C-V conversion circuit 103 has its output fed through a high-pass
filter (HPF) 104 having a cutoff frequency of 10 kHz so that the induction
noise of 2 kHz caused by electrostatic driving is eliminated. The
resulting voltage signal is fed to a synchronous detection circuit 105,
which implements the synchronous detection for the signal based on the
frequency and phase of the oscillation circuit (oscillator) 101. The
detection output is fed through a low-pass filter (LPF) 106, with its
output representing the variation of static capacitance between the three
elastic arms 2a, 2b and 2c of the vibrator 1 and the vertical electrodes
5a, 5b and 5c in their vertical direction.
The vibrator 1 which is made of low-resistivity silicon is conceived to be
electrically conductive for the alternating voltage with respect to the
impedance component of the static capacitance between the elastic arms
2a,2b and 2c of the vibrator 1 and the vertical electrodes 5a,5b and 5c.
Accordingly, the capacitance variations of the three elastic arms 2a,2b
and 2c are coupled at the signal lead-out section 100 and fed by being
summed to the C-V conversion circuit 103.
Among the horizontal electrodes 4a,4b,4c and 4d, the electrodes 4a and 4c
that are in jumper connection and the electrodes 4b and 4d that are in
jumper connection are connected to an oscillation circuit 120 and a phase
inverting circuit 121, respectively, by way of d.c. component cutting
capacitors and voltage dividing resistors (not shown). The oscillation
circuit 120 generates a voltage having a frequency of 70 kHz and amplitude
of 5 volts.
Due to the spacing with a 20-.mu.m gap between the horizontal electrodes
4a,4b,4c and 4d and the three elastic arms 2a,2b and 2c of vibrator, there
emerge the static capacitance across the gaps, which is converted into a
voltage value by the C-V conversion circuit 103.
The C-V conversion circuit 103 has its output fed through a HPF 109 having
a cutoff frequency of 10 kHz, and subsequently rendered the synchronous
detection by a synchronous detection circuit 110 based on the frequency
and phase of the oscillation circuit (oscillator) 120. The detection
output is fed through a LPF 111, with its output representing the
variation of static capacitance between the horizontal electrodes 4a, 4b,
4c and 4d and the three elastic arms 2a,2b and 2c of the vibrator 1 in
their horizontal direction. The paired horizontal electrodes 4a and 4c and
4b and 4d are connected to amplify/d.c. merge circuits 118 and 119,
respectively, by way of voltage dividing resistors (not shown).
The LPF 111 has its output fed through a +90.degree. phase shift circuit
112 and a PLL circuit 113, which is made up of a voltage-controlled
oscillator (VCO) 116, a phase detection circuit 114 which compares the
vibration phase of the vibrator with the oscillation phase of the VCO 116,
and a loop filter 115. The PLL circuit 113 has its output amplified by the
amplify/d.c. merge circuits 118 and 119, by which a d.c. voltage is
superimposed, and consequently there emerges an electrostatic attraction
force between the elastic arms 2a,2b and 2c of the vibrator 1 and the
horizontal electrodes 4a, 4b, 4c and 4d. The horizontal electrodes 4b and
4d have the application of the alternating voltage of opposite phase
relative to the horizontal electrodes 4a and 4c provided by a phase
inverting circuit 117.
With respect to the output of the amplify/d.c. merge circuit 119 and the
input of the C-V conversion circuit 103, i.e., the vibration of vibrator 1
in the horizontal direction, the output of the C-V conversion circuit 103
always lags by 90.degree. by the presence of the +90.degree. phase shift
circuit 112. That is, the vibration of vibrator 1 in the horizontal
direction lags by 90.degree., which phase is kept locked by the PLL
circuit 113. Consequently, the vibrator 1 vibrates continuously at its
inherent resonant frequency and in a resonant state in the horizontal
direction (the actual displacement lags by 90.degree. behind the drive
phase of the vibrator).
In consequence, the vibration continues in the resonant state, which yields
the maximum displacement, even if the resonant frequency inherent to the
vibrator 1 varies due to a small variation of its dimensions caused by an
external temperature variation. The PLL circuit 113 has its output used
for the sync signal of a synchronous detection circuit 107. Accordingly,
the circuit 107 detects only the component that is in-phase with the
output of the PLL circuit 113 out of the capacitance variation of the
vibrator 1 in the vertical direction, and the detection output is treated
by the following LPF 108 to deliver an intended output. In this case, the
output phase is locked by the PLL circuit 113 and thus is not disturbed by
the operation of the synchronous detection circuit 107, whereby a stable
output is obtained.
When the silicon gyroscope is subjected to an angular velocity along the
longitudinal direction of the vibrator 1, the Coriolis force acts on the
elastic arms 2a,2b and 2c of vibrator 1 which are vibrating in the
horizontal direction.
A Coriolis force Fc resulting from an angular velocity w (vector) exerted
on a mass m which is moving at a velocity V (vector) is expressed as Fc=2
m(Vxw), where x indicates the vector product. For example, when the
vibrator 1 is vibrating horizontally and is subjected to an angular
velocity about its longitudinal axis, the Coriolis force acts on it in the
vertical direction. Therefore, the vibrator 1 has a displacement in the
vertical direction in synchronism with the horizontal vibration. The value
of displacement is proportional to the exerted angular velocity.
The vertical displacement of the vibrator 1 is proportional to the
variation of static capacitance created in unison with the vertical
electrodes 5a,5b and 5c. Accordingly, by detecting the static capacitance
variation of vertical direction which is in-phase with the horizontal
vibration of the vibrator 1, the magnitude of angular velocity and its
direction (turning direction) exerted on the silicon gyroscope can be
known.
The foregoing behavior of the silicon gyroscope which implements the
horizontal drive and vertical detection holds also in the case of an
alternative arrangement for vertical drive and horizontal detection, with
the associated circuits being interchanged.
Next, the inventive method of driving the vibrator 1 will be explained in
connection with FIG. 7 and FIG. 8 showing the outputs of the amplify/d.c.
merge circuits 118 and 119 shown in FIG. 6. These graphs have a horizontal
axis for the time and a vertical axis for the output voltage, which gives
the reference potential of 0 volt at the intersection with the time axis.
Since the circuit arrangement is designed such that the C-v conversion
circuit 103 has its potential seen from the vibrator 1 being equivalent to
the reference potential, the potential of vibrator seen from the reference
potential is conceived to be zero volt.
In FIG. 7 and FIG. 8, the vibrator 1 has the reference potential, while the
horizontal electrodes 4a,4b,4c and 4d have their potential varying in
sinusoidal fashion, resulting in a potential difference emerging between
the vibrator 1 and the horizontal electrodes 4a,4b,4c and 4d. Although the
horizontal electrodes 4a,4b,4c and 4d actually have the superimposed
carrier wave of 70 kHz provided by the oscillation circuit 120 and a phase
inverting circuit 121, this frequency component is insignificant for the
explanation of circuit operation and is not shown in FIGS. 7 and 8.
The horizontal electrodes 4a and 4c have their potential consisting of the
alternating voltage and a superimposed positive d.c. voltage as shown in
FIG. 7, while the horizontal electrodes 4b and 4d have their potential
consisting of the alternating voltage of an inverted phase and the same
superimposed positive d.c. voltage as shown in FIG. 8.
The vibration of the elastic arms 2a,2b and 2c of the vibrator 1 caused by
the produced electric field will be explained in connection with FIG. 9
and FIG. 10. These figures schematize the relation among the elastic arms
2a,2b and 2c of the vibrator 1, the horizontal electrodes 4 which are
shown by double-frame blocks, and the vertical electrodes (not shown).
In FIG. 9, labels ON and OFF indicate the voltage application to the
horizontal electrodes at time point t=T in FIGS. 7 and 8. Specifically,
according to the voltage application to the horizontal electrodes 4a and
4c shown in FIG. 7, these electrodes have the maximum potential difference
from the vibrator 1 at time point t=T, i.e., ON state. Similarly, the
horizontal electrodes 4b and 4d have the minimum potential difference from
the vibrator 1 at this time point shown in FIG. 8, i.e., OFF state.
In FIG. 9, the horizontal electrodes 4a and 4c are ON and the horizontal
electrodes 4b and 4d are OFF at time point t=T, and the resulting electric
field causes the outer elastic arms 2a and 2c of the vibrator 1 to move to
the left and the middle elastic arm 2b to move to the right. At this time,
if the gyroscope is subjected to a clockwise angular velocity on the
drawing, the Coriolis force acts on the arms 2a, 2b and 2c as indicated by
the arrows in FIG. 10, causing the elastic arms to have displacements in
the directions indicated by the arrows. Accordingly, the elastic arms
2a,2b and 2c have vertical displacements, resulting in a narrowed gap
between the arms 2a and 2c and the vertical electrodes 5a and 5c (not
shown) and a widened gap between the arm 2b and the vertical electrode 5b
(not shown). Consequently, the static capacitance varies, and the angular
velocity can be evaluated by measuring the variation of static
capacitance.
In case the gyroscope is subjected to a counterclockwise angular velocity,
the directions of arrows mentioned in the foregoing explanation are
reversed. The output of synchronous detection circuit which is in-phase
with the vibrator displacement reverses, and the direction of angular
velocity can be discriminated.
FIG. 11 and FIG. 12 schematize the operation of the gyroscope at another
time point which is shifted by .pi. from time point t=T, i.e., the
vibrator 1 is driven by the alternating voltage in opposite phase.
The C-V conversion circuit 103 is designed such that the amplitude of the
carrier wave which is derived from the oscillation circuit 101 is
proportional to the value of static capacitance. For example, when the gap
between the outer elastic arms 2a and 2c of the vibrator 1 and the
vertical electrodes (not shown) narrows as shown in FIG. 10, a resulting
increased static capacitance increases the amplitude of the carrier wave
component which is in-phase with the oscillation circuit 101. Whereas, the
static capacitance of the middle elastic arm 2b decreases. Since the
vertical electrode which confronts the middle elastic arm 2b is provided
by the phase inverting circuit 102 with the carrier wave of opposite phase
relative to the oscillation circuit 101, the carrier wave component of
opposite phase decreases.
As a result, at the output of the C-V conversion circuit 103, the carrier
wave component which is in-phase with the oscillation circuit 101
increases in response to the Coriolis force acting at the timing shown in
FIGS. 9 and 10. The output of the synchronous detection circuit 107 is fed
through the LPF 108, and the output of positive polarity, for example, is
delivered.
At a counterclockwise angular velocity, the direction of Coriolis force
shown in FIGS. 10 and 12 reverses. The LPF 108 delivers the output of
negative polarity, and the direction of angular velocity can be
discriminated.
FIG. 13 shows a silicon gyroscope based on the second embodiment this
invention, and FIG. 14 shows by a plan view the dimensions of the vibrator
1 of this embodiment. The vibrator 1 fabricated to have these dimensions
operates at a resonant frequency of about 2 kHz. FIG. 15 and FIG. 16 are
cross-sectional views taken along the line 15--15 and line 16--16,
respectively, of FIG. 14.
The silicon gyroscope of the second embodiment differs from that of the
preceding first embodiment only in the formation of vertical electrodes 6
in pairs (6a,6b; 6c,6d; 6e,6f) in correspondence to the elastic arms 2a,2b
and 2c of the vibrator 1.
FIG. 17 shows by a block diagram the circuit arrangement for driving and
detection of the silicon gyroscope of the second embodiment shown in FIG.
13. The oscillation circuit 101 generates an alternating voltage having a
frequency of 50 kHz and amplitude of 5 volts, and the voltage is applied
to the horizontal electrodes 4a and 4c. The phase inverting circuit 102
produces a carrier wave of opposite phase relative to the voltage provided
by the oscillation circuit 101, and it is applied to the horizontal
electrodes 4b and 4d.
The horizontal electrodes 4a, 4b, 4c and 4d are spaced out with a gap of 20
.mu.m from the elastic arms 2a,2b and 2c of the vibrator 1, so that a
static capacitance emerges across the gaps, which is converted into a
voltage value by the C-V conversion circuit 103.
The C-V conversion circuit 103 has its output fed through the HPF 104
having a cutoff frequency of 10 kHz, and the resulting voltage signal is
fed to the synchronous detection circuit 105, which implements the
synchronous detection for the signal based on the frequency and phase of
the oscillation circuit 101. The detection output is fed through the LPF
106, with its output representing the variation of static capacitance
between the horizontal electrodes 4a, 4b, 4c and 4d and the three elastic
arms 2a,2b and 2c of the vibrator 1 in their horizontal direction.
The vibrator 1 which is made of low-resistivity silicon is conceived to be
electrically conductive for the alternating voltage with respect to the
impedance component of the static capacitance between the elastic arms
2a,2b and 2c of the vibrator land the horizontal electrodes 4a to 4d.
Accordingly, the capacitance variations of the three elastic arms 2a, 2b
and 2c are coupled at the signal lead-out section 100 and fed by being
summed to the C-V conversion circuit 103.
Among the vertical electrodes 6a, 6b, 6c, 6d, 6e and 6f, the electrodes 6a
and 6e that are in jumper connection and the electrodes 6b and 6f that are
in jumper connection are connected to the oscillation circuit 120 by way
of voltage dividing resistors (not shown). The vertical electrodes 6c and
6d are connected to the phase inverting circuit 121 by way of voltage
dividing resistors (not shown).
The oscillation circuit 120 generates a voltage having a frequency of 70
kHz and amplitude of 5 volts. Due to the spacing with a 20-.mu.m gap
between the vertical electrodes 6a,6b,6c,6d,6e and 6f and the three
elastic arms 2a,2b and 2c of vibrator, there emerge the static capacitance
across the gaps, which is converted into a voltage value by the C-V
conversion circuit 103.
The C-V conversion circuit 103 has its output fed through the HPF 109
having a cutoff frequency of 10 kHz, and subsequently rendered the
synchronous detection by the synchronous detection circuit 110 based on
the frequency and phase of the oscillator 120. The detection output is fed
through the LPF 111, with its output representing the variation of static
capacitance between the vertical electrodes 6a,6b,6c,6d,6e and 6f and the
three elastic arms 2a,2b and 2c of the vibrator 1 in their vertical
direction.
The paired vertical electrodes 6a and 6e and 6d and 6f are connected to the
amplify/d.c. merge circuits 118 and 119, respectively, by way of d.c.
component cutting capacitors and voltage dividing resistors (not shown).
The vertical electrodes 6c and 6d are connected to the amplify/d.c. merge
circuits 132 and 131, respectively.
The LPF 111 has its output fed through the +90.degree. phase shift circuit
112 and the PLL circuit 113, which is made up of the voltage-controlled
oscillator (VCO) 116, the phase detection circuit 114 which compares the
vibration phase of the vibrator with the oscillation phase of the VCO 116,
and the loop filter 115, so that there is always a phase difference of
+90.degree. between the input and output of the PLL circuit 113, i.e.,
between the output of the +90.degree. phase shift circuit 112 and the
output of the VCO 116. The PLL circuit 113 has its output amplified by the
amplify/d.c. merge circuits 131,132,134 and 135, by which a d.c. voltage
is superimposed, and consequently there emerges an electrostatic
attraction force between the elastic arms 2a,2b and 2c of the vibrator 1
and the vertical electrodes 6a,6b,6c,6d,6e and 6f.
With respect to the outputs of the amplify/d.c. merge circuits 132 and 135
and the input of the C-V conversion circuit 103, i.e., the vibration of
vibrator 1 in the vertical direction, the output of the C-V conversion
circuit 103 always lags by 90.degree. by the presence of the +90.degree.
phase shift circuit 112. That is, the vibration of vibrator 1 in the
vertical direction lags by 90.degree., which phase is kept locked by the
PLL circuit 113. Consequently, the vibrator 1 vibrates continuously at its
inherent resonant frequency and in a resonant state in the vertical
direction (the actual displacement lags by 90.degree. behind the drive
phase of the vibrator).
Next, the inventive method of voltage application to the vertical
electrodes will be explained in connection with FIG. 18 through FIG. 21
showing the outputs of the amplify/d.c merge circuits 134,135,131 and 132,
respectively, shown in FIG. 17. These graphs have a horizontal axis for
the time and a vertical axis for the output voltage, which gives the
reference potential of 0 volt at the intersection with the time axis.
Since the circuit arrangement is designed such that the C-V conversion
circuit 103 has its potential seen from the vibrator 1 being equivalent to
the reference potential, the potential of vibrator seen from the reference
potential is conceived to be zero volt.
In FIGS. 18-21, the vibrator 1 has the reference potential and it is
subjected to the electric fields attributable to the potentials of the
vertical electrodes 6a,6b,6c,6d,6e and 6f varying in sinusoidal fashion.
The vertical electrode pairs 6a and 6b and 6e and 6f which confront the
outer elastic arms 2a and 2c of the vibrator 1 have opposite phases and
also have opposite polarities of superimposed d.c. voltage. Similarly, the
vertical electrodes 6c and 6d which confront the middle arm 2b have
opposite phases and d.c. polarities as shown in FIGS. 20 and 21.
By the application of these voltages, the elastic arms 2a,2b and 2c of the
vibrator 1 have the exertion of electrostatic force of the confronting
electrodes at time point t=T in FIG. 18 through FIG. 21 as follows.
arms 2a 2b 2c
electrodes 6a 6b 6c 6d 6e 6f
a.c. component positive negative negative positive positive
negative
maximum maximum minimum minimum maximum
maximum
d.c. component positive negative negative positive positive
negative
electrostatic maximum minimum maximum
force
Accordingly, the outer elastic arms 2a and 2c have the maximum
electrostatic force of the electrodes, while the middle arm 2b has the
minimum force of the electrode.
FIG. 22 and FIG. 23 schematize the movement of the elastic arms 2a,2b and
2c and the Coriolis force acting on them in the presence of the
electrostatic force and a clockwise angular velocity. The figures show
that the electrostatic force of the elastic arms 2a and 2c is maximum in
the vertical direction, while the electrostatic force of the elastic arm
2b is minimum.
The Coriolis force acts on the elastic arms as shown in FIG. 23, causing
the static capacitance to vary, and the angular velocity can be evaluated
by measuring the variation of static capacitance.
In case the gyroscope is subjected to a counterclockwise angular velocity,
the directions of arrows mentioned in the foregoing explanation are
reversed. The output of synchronous detection circuit which is in-phase
with the vibrator displacement reverses, and the direction of angular
velocity can be discriminated.
FIG. 24 and FIG. 25 schematize the operation of the gyroscope at another
time point which is shifted by .pi. from time point t=T, i.e., the
vibrator 1 is driven by the alternating voltage in opposite phase.
The C-V conversion circuit 103 is designed such that the amplitude of the
carrier wave which is derived from the oscillation circuit 101 is
proportional to the value of static capacitance. For example, when the
elastic arms 2a and 2c of the vibrator 1 have displacements due to the
Coriolis force shown in FIG. 23, the amplitude of the carrier wave
component which is in-phase with the oscillation circuit (oscillator) 101
decreases, while the carrier wave component which is in-phase with the
phase inverting circuit 102, i.e., opposite in phase from the oscillation
circuit 101, increases.
As a result, at the output of the C-V conversion circuit 103, the carrier
wave component which is opposite in phase from the oscillation circuit 101
increases in the presence of a clockwise angular velocity and the Coriolis
force shown in FIG. 23. The output of the synchronous detection circuit
107 is fed through the LPF 108, and the output of negative polarity, for
example, is delivered.
At a counterclockwise angular velocity, the direction of Coriolis force
shown in FIGS. 23 and 25 reverses. The LPF 108 delivers the output of
positive polarity, and the direction of angular velocity can be
discriminated.
The C-V conversion circuit 103, which converts the input of high-impedance
static capacitance into a voltage value, is susceptible to electromagnetic
noises or the like. The variation of static capacitance caused by the
Coriolis force is as extremely small as several tens aF, and it can
possibly be hidden in external electromagnetic noises instead of being
detected.
The preceding first embodiment is also designed to drive the vibrator 1 to
vibrate by exerting the electrostatic force and detect the displacement of
elastic arms 2a,2b and 2c of the vibrator 1 caused by the Coriolis force
in terms of the variation of static capacitance. Because of the same
frequency of the drive operation and of the variation of static
capacitance caused by the Coriolis force, induction noises created at
driving can possibly leak into the C-V conversion circuit 103.
Whereas, the second embodiment is designed to divide the driving electrodes
into multiple electrodes (a pair of vertical electrodes for each elastic
arm 2 in this embodiment), with a.c. components of opposite phases being
applied thereto, so that the influence of drive voltage components on the
elastic arms 2a,2b and 2c is reduced. For example, the preceding table of
electrostatic force reveals that for the elastic arm 2a in FIG. 13, the
a.c. components of the vertical electrodes 6a and 6b cancel each other as
a result of summation. This fact also holds in the cases of remaining
vertical electrodes 6c,6d,6e and 6f corresponding to the middle and
rightmost elastic arms 2b and 2c.
Based on the dividing of driving electrodes and application of a.c.
components of opposite phases to the divided electrodes, the electrostatic
force comparable to the case of the undivided electrode can be obtained
virtually without a loss of electrostatic force per unit area attributable
to division, whereby it becomes possible to preclude the induction noise
from being fed to the C-V conversion circuit 103.
That is, based on the dividing of the driving electrodes and application of
a.c. components of opposite phases and d.c. components of opposite
polarities to the divided electrodes so that the influences of electrodes
cancel each other, the induction noise arising on the part of the
detecting electrodes is minimized. Based on such rearrangement of
electrodes for the same sensing device, it can achieve an enhanced
sensitivity of detection and a high s/n ratio. Based on the dividing of
the driving electrodes, it becomes possible to produce an electrostatic
force comparable to the case of the undivided electrode at the same supply
voltage virtually without a loss of electrostatic force to be exerted on
the vibrator 1. It achieves the inherent function of driving the vibrator
1 identically to the case of the undivided electrode. Moreover, by finely
adjusting the amplitude of the a. c. component which causes the induction
noise, the second embodiment effectuates the cancellation of influence
more than the preceding first embodiment.
The present invention is not confined to the foregoing embodiments, but
various modifications are possible when necessary. For example, a variant
design is such that the elastic arms of vibrator are driven in the
vertical direction, and their displacements caused by the Coriolis force
which acts on the electrodes in the horizontal direction are detected.
Next, the third and fourth embodiments of this invention will be explained
with reference to FIG. 26 through FIG. 34.
FIG. 26 is a plan view of the silicon gyroscope of the third embodiment,
and FIG. 27 and FIG. 28 are cross-sectional views taken along the line
27--27 and line 28--28, respectively, of FIG. 26.
The main body of silicon gyroscope of the third embodiment shown in FIG. 26
through FIG. 28 is made up of a vibrator 501, a base plate 503, and
vertical electrodes 504. The vibrator 501 which is made of silicon
consists of a root section 501a and three elastic arms 502a,502b and 502c
which are parallel to each other and separated by two notches running from
the root section 501a. The vibrator 501 is joined at its end of root
section 501a to the base plate 503 of glass by means of anode plate
bonding technique or the like.
The elastic arms 502a,502b and 502c are dimensioned to have a thickness d
and a width W of notches between elastic arms related in terms of the
value of W/d to be in the range from 1 to 0.02. If the W/d value is set 1
or smaller, the Q value becomes 1000 or greater as shown in FIG. 34, and
making the W/d value smaller than 0.02 is technically difficult.
Specifically, in the case of using a silicon wafer with a thickness d of
300 .mu.m, the notch width w is preferably set within the range of 30 to
150 .mu.m.
The vertical electrodes 504 which are made of chrome in this embodiment
include driving electrodes 504A, drive-synchronous (-sync) electrodes 504B
and Coriolis force detecting electrodes 504C, and these electrodes are
laid in a groove 503a, which is formed on the upper surface of the base
plate 503, by being parallel to the layout plane of the elastic arms
502a,502b and 502c and close to the arms.
Specifically, vertical electrodes provided for the elastic arm 502a are two
driving electrodes 504Aa and 504Ab and a drive-sync electrode 504Ba which
is located between the electrodes 504Aa and 504Ab for taking the RFE
output for synchronous detection of the elastic arm 502a, as shown in
FIGS. 26 and 28. Similarly, the elastic arm 502b is provided with two
driving electrodes 504Ac and 504Ad and a drive-sync electrode 504Bb which
is located between the electrodes 504Ac and 504Ad for taking the RFE
output for synchronous detection of the elastic arm 502b, and the elastic
arm 502c is provided with two driving electrodes 504Ae and 504Af and a
drive-sync electrode 504Bc which is located between the electrodes 504Ae
and 504Af for taking the RFE output for synchronous detection of the
elastic arm 502c.
Among other vertical electrodes, Coriolis force detecting electrodes 504Cb
and 504Cc are laid in the groove 503a by being located between the elastic
arms 502a and 502b and between the elastic arms 502b and 502c,
respectively, and Coriolis force detecting electrodes 504Ca and 504Cd are
laid in the groove 503a by being located on the outer sides of the elastic
arms 502a and 502c, respectively. The vertical electrodes of this
invention are made of chrome.
The vibrator 501 is adapted to vibrate at its elastic arms 502a, 502b and
502c by being secured at its root section 501a. The vibration of the
elastic arms 502a,502b and 502c propagated to the root section 501a is so
weak that it does not virtually cause the end of root section 501a to
vibrate. According to this invention, the end of root section 501a is
joined to the base plate 503 so that it does not affect the vibration of
the elastic arms 502a,502b and 502c.
The vibrator 501 can have its Q value, which indicates the degree of
resonance, increased by having a narrower gap among the elastic arms
502a,502b and 502c as mentioned previously. The Q value represents the
output (vibration amplitude of the elastic arms) with respect to the
input, and the greater the Q value, the larger is the resonance amplitude,
as shown in FIG. 34. It signifies that a silicon gyroscope, which has a
small Q value and necessitates a drive voltage of 100 volts for example,
will be able to operate at 15 volts or lower if the Q value is raised. A
silicon gyroscope based on a three-arm tuning fork has its Q value raised
by narrowing the arm gap, provided that the thickness of substrate is kept
constant in both the horizontal and vertical directions.
The silicon gyroscope of this embodiment also can be designed to operate at
a lower drive voltage by providing the elastic arms 502a, 502b and 502c
with a narrow gap of 30 to 150 .mu.m and using a silicon wafer of
300-.mu.m thickness.
The vibrator 1 can vibrate freely in the vertical and horizontal
directions, so that it vibrates without twisting by being driven in the
horizontal or vertical direction, allowing the detection of the
displacement of vibrator 501 caused by the Coriolis force in the direction
orthogonal to the drive direction based on the detection of the component
orthogonal to the drive direction.
FIG. 29 shows by a block diagram an embodiment of the circuit arrangement
for driving and detection of this silicon gyroscope. An oscillation
circuit 601 generates an alternating voltage having a frequency of 50 kHz
and amplitude of 5 volts, and it is applied to the Coriolis force
detecting electrodes 504Cb and 504Cd. A phase inverting circuit 602
produces a carrier wave of opposite phase relative to the voltage provided
by the oscillation circuit 601, and the voltage is applied to the Coriolis
force detecting electrodes 504Ca and 504Ce.
The Coriolis force detecting electrodes 504C out of the vertical electrodes
504 are spaced out with a gap of 20 .mu.m from the elastic arms 502a,502b
and 502c of the vibrator 501, so that a static capacitance emerges across
the gaps, which is converted into a voltage value by a C-V conversion
circuit 603.
The C-V conversion circuit 603 has its output fed through a HPF 604 having
a cutoff frequency of 10 kHz so that the induction noise of 2 kHz caused
by electrostatic driving is eliminated. The resulting voltage signal is
fed to a synchronous detection circuit 605, which implements the
synchronous detection for the signal based on the frequency and phase of
the oscillation circuit (oscillator) 601. The detection output is fed
through a LPF 606, with its output representing the variation of static
capacitance between the Coriolis force detecting electrodes 504C and the
three elastic arms 502a, 502b and 502c of the vibrator 501 in their
horizontal direction.
The vibrator 501 which is made of low-resistivity silicon is conceived to
be electrically conductive for the alternating voltage with respect to the
impedance component of the static capacitance between the elastic arms
502a, 502b and 502c of the vibrator 501 and the driving electrodes 504A.
Accordingly, the capacitance variations of the three elastic arms 502a,
502b and 502c are coupled at the signal lead-out section 600 and fed by
being summed to the C-V conversion circuit 603.
Among the drive-sync electrodes 504B, the electrodes 504Ba and 504Bc that
are in jumper connection are connected to a phase inverting circuit 621,
and the electrode 504Bb is connected to an oscillation circuit 620 which
generates a voltage having a frequency of 70 kHz and amplitude of 5 volts.
Due to the spacing with a 20-.mu.m gap between the drive-sync electrodes
504Ba,504Bb and 504Bc and the three elastic arms 502a,502b and 502c of
vibrator 501, there emerge a static capacitance across the gaps, which is
converted into a voltage value by the C-V conversion circuit 603.
The C-V conversion circuit 603 has its output fed through a HPF 609 having
a cutoff frequency of 10 kHz, and subsequently rendered the synchronous
detection by a synchronous detection circuit 610 based on the frequency
and phase of the oscillation circuit (oscillator) 620. The detection
output is fed through a LPF 611, with its output representing the
variation of static capacitance between the drive-sync electrodes 504B and
the three elastic arms 502a,502b and 502c of the vibrator 501 in their
vertical direction. The driving electrode set 504Ac and 504Ad and the
other driving electrode set 504Aa,504Ab,504Ae and 504Af are connected to
amplify/d.c. merge circuits 618 and 619, respectively.
The LPF 611 has its output fed through a +90.degree. phase shift circuit
612 and a PLL circuit 613, which is made up of a voltage-controlled
oscillator (VCO) 616, a phase detection circuit 614 which compares the
vibration phase of the vibrator with the oscillation phase of the VCO 616,
and a loop filter 615. The PLL circuit 613 has its output amplified by the
amplify/d.c. merge circuits 618 and 619, by which a d.c. voltage is
superimposed, and consequently there emerges an electrostatic attraction
force between the elastic arms 502a,502b and 502c of the vibrator 501 and
the driving electrodes 504A. The driving electrodes 504Ac and 504Ad have
the application of the alternating voltage of opposite phase relative to
the driving electrodes 504Aa, 504Ab, 504Ae and 504Af provided by a phase
inverting circuit 617.
With respect to the output of the amplify/d.c. merge circuit 619 and the
input of the C-V conversion circuit 603, i.e., the vibration of vibrator
501 in the vertical direction, the output of the C-V conversion circuit
603 always lags by 90.degree. by the presence of the +90.degree. phase
shift circuit 612. That is, the vibration of vibrator 501 in the vertical
direction lags by 90.degree., which phase is kept locked by the PLL
circuit 613. Consequently, the vibrator 501 vibrates continuously at its
inherent resonant frequency and in a resonant state in the vertical
direction (the actual displacement lags by 90.degree. behind the drive
phase of the vibrator).
In consequence, the vibration continues in the resonant state, which yields
the maximum displacement, even if the resonant frequency inherent to the
vibrator 501 varies due to a small variation of its dimensions caused by
an external temperature variation. The PLL circuit 613 has its output used
for the sync signal of a synchronous detection circuit 607. Accordingly,
the circuit 607 detects only the component that is in-phase with the
output of the PLL circuit 613 out of the capacitance variation of the
vibrator 501 in the horizontal direction, and the detection output is
treated by the following LPF 608 to deliver an intended output. In this
case, the output phase is locked by the PLL circuit 613 and thus is not
disturbed by the operation of the synchronous detection circuit 607,
whereby a stable output is obtained.
When the silicon gyroscope is subjected to an angular velocity along the
longitudinal direction of the vibrator 501, the Coriolis force acts on the
elastic arms 502a, 502b and 502c of vibrator 501 which are vibrating in
the vertical direction. For example, when the vibrator 501 is vibrating
vertically and is subjected to an angular velocity about its longitudinal
axis, the Coriolis force acts on it in the horizontal direction.
Therefore, the vibrator 501 has a displacement in the horizontal direction
in synchronism with the vertical vibration. The value of displacement is
proportional to the exerted angular velocity.
The horizontal displacement of the vibrator 501 is proportional to the
variation of static capacitance created in unison with the Coriolis force
detecting electrodes 504C. Accordingly, by detecting the static
capacitance variation of horizontal direction which is in-phase with the
vertical vibration of the vibrator 501, the magnitude of angular velocity
and its direction (turning direction) exerted on the silicon gyroscope can
be known.
However, the silicon gyroscope of this embodiment, which is intended to
narrow the gap among the elastic arms 502a,502b and 502c, does not have in
the gaps the provision of horizontal electrodes for detecting the
capacitance variation. It is otherwise designed to detect the displacement
in the horizontal direction from the variation of coincidence area between
the Coriolis force detecting electrodes 504C and the confronting elastic
arms 502a,502b and 502c.
The silicon gyroscope of this embodiment has the vertical drive-sync
electrodes 504B separately from the driving electrodes 504A. On this
account, it does not need to divide voltages by placing voltage dividing
resistors on the routes of the oscillation circuit 620, phase inverting
circuit 621 and amplify/d.c. voltage merge circuits 618 and 619.
Accordingly, the power voltage is utilized effectively, and thus it
becomes possible to drive the silicon gyroscope at a lower power voltage.
The drive-sync electrodes 504B release the RFE output for synchronous
detection, based on which the displacement in the vertical direction can
be detected.
FIG. 30 shows by a block diagram another embodiment of the circuit
arrangement for driving and detection of this silicon gyroscope. This
circuit arrangement differs from the previous embodiment in that d.c.
voltages of opposite polarities are applied to a pair of driving
electrodes 504A which correspond to each elastic arm 502.
Among the driving electrodes 504A, the electrode pairs 504Ab and 504Af and
504Aa and 504Ae are connected to the amplify/d.c. voltage merge circuits
634 and 635, respectively, and the electrodes 504Ad and 504Ac are
connected to the amplify/d.c. voltage merge circuits 632 and 631,
respectively. The LPF 611 has its output fed through the +90.degree. phase
shift circuit 612 and the PLL circuit 613, which is made up of a VCO 616,
a phase detection circuit 614 which compares the vibration phase of the
vibrator with the oscillation phase of the VCO 616, and a loop filter 615,
so that there is always a phase difference of +90.degree. between the
input and output of the PLL circuit 613, i.e., between the output of the
+90.degree. phase shift circuit 612 and the output of the VCO 616. The PLL
circuit 613 has its output amplified by the amplify/d.c. merge circuits
631,632, 634 and 635, by which a d.c. voltage is superimposed, and
consequently there emerges an electrostatic attraction force between the
elastic arms 502a,502b and 502c of the vibrator 501 and the driving
electrodes 504Aa, 504Ab, 504Ac, 504Ad, 504Ae and 504Af.
With respect to the outputs of the amplify/d.c. merge circuits 632 and 635
and the input of the C-V conversion circuit 603, i.e., the vibration of
vibrator in the vertical direction, the output of the C-V conversion
circuit 603 always lags by 90.degree. by the presence of the +90.degree.
phase shift circuit 612. That is, the vibration of vibrator 501 in the
vertical direction lags by 90.degree., which phase is kept locked by the
PLL circuit 613. Consequently, the vibrator 501 vibrates continuously at
its inherent resonant frequency and in a resonant state in the vertical
direction (the actual displacement lags by 90.degree. behind the drive
phase of the vibrator).
In consequence, the vibration continues in the resonant state, which yields
the maximum displacement, even if the resonant frequency inherent to the
vibrator 501 varies due to a small variation of its dimensions caused by
an external temperature variation. The PLL circuit 613 has its output used
for the sync signal of a synchronous detection circuit 607. Accordingly,
the circuit 607 detects only the component that is in-phase with the
output of the PLL circuit 613 out of the capacitance variation of the
vibrator 501 in the vertical direction, and the detection output is
treated by the following LPF 608 to deliver an intended output. In this
case, the output phase is locked by the PLL circuit 613 and thus is not
disturbed by the operation of the synchronous detection circuit 607,
whereby a stable output is obtained.
Accordingly, the fourth embodiment is designed to divide the driving
electrodes into multiple electrodes (a pair of vertical electrodes for
each elastic arm 502 in this embodiment), with a.c. and d.c. voltage
components of opposite phases and polarities being applied thereto, so
that the influence of drive components on the C-V conversion circuit 603
is reduced. Specifically, for the elastic arm 502a, for example, the a.c.
and d.c. components of the driving electrodes 504Aa and 504Ab cancel each
other as a result of summation. This fact also holds of the remaining
driving electrode pairs 504Ac and 504Ad and 504Ae and 504Af corresponding
to the middle and rightmost elastic arms 502b and 502c, respectively.
Based on the dividing of the driving electrodes 504A corresponding to the
elastic arms 502a,502b and 502c and application of a.c. voltage components
of opposite phases and d.c. voltage components of opposite polarities to
the divided electrodes, the electrostatic force comparable to the case of
the undivided electrode can be obtained virtually without a loss of
electrostatic force per unit area attributable to division, whereby it
becomes possible to preclude the induction noise from being fed to the C-V
conversion circuit 603.
That is, based on the dividing of the driving electrodes and application of
a.c. voltage components of opposite phases and d.c. voltage components of
opposite polarities to the divided electrodes so that their influences
cancel each other, the induction noise arising on the part of the
detecting electrodes is minimized. Based on such rearrangement of
electrodes for the same sensing device, it can achieve an enhanced
sensitivity of detection and a high s/n ratio. Based on the dividing of
the driving electrodes, it becomes possible to produce an electrostatic
force comparable to the case of the undivided electrode at the same supply
voltage virtually without a loss of electrostatic force to be exerted on
the vibrator 501. It achieves the function of driving the vibrator 501
identically to the case of the undivided electrode. Moreover, by finely
adjusting the amplitude of the a.c. component which causes the induction
noise, the influence cancellation effect can be enhanced.
FIG. 31 through FIG. 33 show a silicon gyroscope based on the fourth
embodiment of this invention, of which FIG. 31 is a plan view of the
silicon gyroscope, and FIG. 32 and FIG. 33 are cross-sectional views taken
along the line 32--32 and line 33--33, respectively, of FIG. 31.
The main body of silicon gyroscope of this embodiment is also made up of a
vibrator 501, a base plate 503, and vertical electrodes 504. The vibrator
501 which is made of silicon consists of a root section 501a and three
elastic arms 502a, 502b and 502c which are parallel to each other and
separated by two notches running from the root section 501a.
Vertical electrodes provided for the elastic arm 502a are two driving
electrodes 504Aa and 504Ab, to which are applied an a.c. and d.c. voltage
components of the same amplitudes and opposite phases and polarities, and
a drive-sync electrode 504Ba which is located between the electrodes 504Aa
and 504Ab for taking the RFE output for synchronous detection of the
elastic arm 502a, as shown in FIGS. 31 and 33. Similarly, the elastic arm
502b is provided with two driving electrodes 504Ac and 504Ad having the
application of an a.c. and d.c. voltage components of the same amplitudes
and opposite phases and polarities and a drive-sync electrode 504Bb which
is located between the electrodes 504Ac and 504Ad for taking the RFE
output for synchronous detection of the elastic arm 502b, and the elastic
arm 502c is provided with two driving electrodes 504Ae and 504Af having
the application of an a.c. and d.c. voltage components of the same
amplitudes and opposite phases and polarities and a drive-sync electrode
504Bc which is located between the electrodes 504Ae and 504Af for taking
the RFE output for synchronous detection of the elastic arm 502c.
Among other vertical electrodes, Coriolis force detecting electrodes 504Ce
and 504Cf are laid in the groove 503a by being located between the elastic
arms 502a and 502b and between the elastic arms 502b and 502c,
respectively, and, different from the silicon gyroscope of the previous
third embodiment, horizontal electrodes 505a and 505b are laid on the
outer sides of the elastic arms 502a and 502c by being orthogonal to the
arm layout plane and close to the arms 502a and 502c. The remaining
structure of the gyroscope main body is identical to the silicon gyroscope
of the previous third embodiment.
The silicon gyroscope of this embodiment arranged as explained above has
basically the same operational characteristics and effectiveness as those
of the previous embodiment. In addition, it facilitates the performance
test at mass production, in which the resonant frequency in the horizontal
direction is measured by driving the vibrator 501 horizontally.
As described above, the inventive silicon gyroscope having its vibrator
made of monocrystal silicon detects the angular velocity accurately and
stably in the environment of varying temperature and throughout the
long-term use, and enables the fabricating process for large-scale
production. Owing to the absence of attached members on the elastic arms,
the vibrator can vibrate stably without their influence.
Driving of the vibrator and detection of Coriolis force are implemented in
non-contact fashion against the vibrator, so that the detection of
Coriolis force is virtually rid of error factors. The orthogonality
between the drive vibration and the vibration of detection is accurate
based on the photolithographic processing of the silicon vibrator, so that
the influence of the drive signal on the detection signal can be
prevented.
By narrowing the gap between elastic arms, the inventive silicon gyroscope
has enhanced sensitivity of Coriolis force detection without the need of
supply of a high drive voltage.
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